After a borehole has been drilled into the earth, a string of steel casing is lowered and set therein. One or more drillable float shoes having an upwardly closing check valve are mounted on or near the lower end of the string to prevent back flow. A cement slurry is pumped down the interior of the casing string and out into the borehole via the check valves where it flows up in the annulus outside the casing to a desired level.
It is highly desirable to protect the cement slurry from contamination by the drilling mud as the slurry is being pumped into the well. The usual practice to protect it is to place a first wiper plug ahead of the cement column which provides a separation between the lower end of such column and the mud, and to place a second wiper plug which performs the same function at the top of the column. Each plug typically has a series of upwardly facing elastomer cups or fins whose outer edges engage the inner walls of the casing to provide sliding seals and wipers. When the first plug lands against a float shoe at the bottom of the casing, a passage is opened up through it which enables cement to be pumped into the annulus. Eventually the second plug lands against the first plug as the displacement is completed. The check valves in the float shoes prevent back flow of the cement into the casing during the time that it takes for the cement to set up. During the downward movement the outer edges of the fins of the second plug wipe or scrape the cement off of the inner walls of the casing so that no deposits are left. Once the cement is hardened, the plugs and cement shoes can be drilled out.
The plugs have annular elastomeric cups or fins that are inwardly compressed within an open-bottomed cylindrical basket near the top of the casing and held in the basket by frictional contact against the inner periphery of the basket. The cement wiper plugs are force-fitted within the basket which temporarily reduces their respective outer diameters. A push rod has a lower end in engagement with the upper wiper plug in the basket and the upper end of the push rod is connected to a hydraulically actuated piston such as shown in European Publication No. 0377255 B1published Jan. 19, 1994. The piston and push rod are mounted for downward movement a distance equal to the length of the two cement plugs. The push rod pushes the first plug out from the open lower end of the basket to form the lower end of the column of cement. The push rod pushes the second wiper plug out from the open lower end of the basket to define the upper end of the column of cement.
Upon expulsion of a plug, the plug expands radially outward to its relaxed diameter where the outer edges of its fins engage the inner wall of the casing. The lower plug moves ahead of a column of cement which is being pumped down the running string and out of lateral ports. From there the cement flows through an annular space between the basket and the inner wall of the casing. At the appropriate time, the upper cement wiper plug also is ejected from the basket and launched into the casing at the upper end of the column of cement. This plug expands like the first one to provide a moving seal that prevents contamination of the upper end of the cement column. The operation of the launcher is controlled from a remote location and the plugs do not require any reloading.
Some cement heads including an upper removable cap and may be loaded with another wiper plug after the launching of a plug. The cap on the upper end of the cement head is removed for reloading of a plug and then replaced after loading. However, the reloading of a plug into the cement head is time consuming and cumbersome with several workmen sometimes required for reloading the cement head with a new plug.
It has been found in many instances that more than two wiper plugs are desirable for multiple interfaces between different fluids. When more than two wiper plugs are utilized, the length of each stroke of the piston is equal to the length of the ejected wiper plug. For example, if four cement wiper plugs, each with a length of fifteen (15) inches, are utilized, the total stroke of the piston would have to be five (5) feet in order to eject or discharge all of the plugs from the lower end of the basket. The basket would also be at least five (5) feet in length in order to receive the plugs. Vertical space on the floor of a drilling rig or derrick is at a premium and is needed for a variety of operations. The cylinder which receives the piston above the basket has a pressurized hydraulic chamber over the piston and would extend substantially over five (5) feet above the basket.
It is an object of the present invention to provide a hydraulic cement launcher particularly adapted for three or more wiper plugs with the launcher being of a minimum height above the wiper plugs.
A further object of the invention is the provision of such a wiper plug launcher in which the basket receiving the wiper plugs forms a fluid pressurized cylinder for receiving actuating hydraulic fluid for ejecting the wiper plugs.